One of the first things many of us notice when we meet someone is what sort of perfume or aftershave they're wearing.
That was certainly the case when I met up with my friend Kerrie the other week and asked what the sweet-smelling scent that surrounded her was.
"Midnight Fantasy by Britney Spears," she replied.
Now, we can all obviously appreciate Britney as pop royalty, but I was surprised such an arresting scent would have a celebrity's name attached to it.
Not long ago, fragrances were associated with well-established, fashionable names such as Ralph Lauren or couture brands like Chanel.
But the number of celebrity perfumes on the market has rocketed in the last decade.
Why? Jennifer Lopez.
"Glow changed everything," says Chandler Burr, the former scent critic for the New York Times and author of several books on the subject.
He credits Lopez's first fragrance, which was released in 2002, with triggering a deluge of deodorants.
"Elizabeth Taylor was one of the first [to have her own scent], but Glow kicked the whole thing into overdrive," he says.
Lopez had sparked what Jezebel brilliantly described as the "scentocalpyse".
Suddenly, everyone and their goldfish had a scent of their own. And they sold by the truckload.
"Brands can see a huge surge in sales and awareness when a celebrity face resonates with their audience," says Gill Smith, managing director of The Perfume Shop.
She cites Beyonce and Ariana Grande's ranges as some of the store's most popular products.
So, what is the appeal of celebrity fragrances? "Identification and intimacy," says Burr.
"Scent is an affordable unit of a star. Assuming the celebrity has actually been involved in its creation, a scent constitutes identification with that star viscerally and intimately. It is, in a small way, meeting them."
Perfumes ideally have to match the celebrity's image while also appealing to their core audience (which explains why pop stars with younger fanbases have fragrances with sugary sweet smells).
Smith says: "We all still aspire to be more like our idols and connecting through a fragrance is one way of doing that.
"Britney Spears has stood the test of time. Diehard fans who wore Britney Fantasy over 12 years ago are still coming to us to try her new fragrances."
This is probably a good moment for me to make a confession. And, before I start, it's not something I'm proud of, ok?
It was August 2014. I'd been to Spain on holiday and was in the duty free section of Barcelona Airport.
Early, bored and trying to get rid of the last of my euros, I was spraying various aftershaves up and down my arm when one caught my nose.
I looked at the bottle and was surprised to see it was something called The Secret by Antonio Banderas.
So I bought it, and was duly mocked by my friends for having spent money on something called The Secret by Antonio Banderas.
But it did make me realise that, if something smells good, it will sell regardless.
For celebrities who don't want to release a fragrance under their own name, the mere act of endorsing an existing brand can have a huge impact.
"Johnny Depp as the face of Dior Sauvage has helped to drive awareness for that fragrance," Smith says. "Eighteen months after launch it's still one of our top 10 sales every day.
"Other examples include Gigi Hadid with Tommy Girl; Jared Leto with Gucci; and Estee Lauder Modern Muse with Kendall Jenner."
She adds: "More recently the announcement of Guerlain working with Angelina Jolie has given a more traditional fragrance house a celebrity boost."
The stigma around celebrity fragrances may have faded over the last few years - but now the sales are fading as well.
It's a decline that started several years ago in the US.
"We saw it in 2008 right after the crash, and it's now a given in the US industry that the celebrity market has collapsed - or at least hugely shrunk," Burr explains.
"Rihanna and a few others have scents that are doing well, but it's nothing like before."
Consumers in the UK appear to be turning their noses up at them too.
Figures released by market research group National Purchase Diary (NPD) show sales of celebrity fragrances declined by £12m in the UK last year - a drop of 22%.
But the fragrance market as a whole actually grew by 1.4% - so it's not that people stopped buying perfume, just that they're less drawn to celebrities.
Just look at the sales of couture brands like Prada, Chanel and Dior, which collectively saw a 6% increase last year.
"The decline in celebrity perfumes is something we noticed in the past two years - it's not something we see normally," says Teresa Fisher, senior account manager in UK Beauty at NPD.
That drop, she points out, could partly be down to fewer launches.
"The market was very healthy a few years ago because there were a lot of celebrity fragrances around," Fisher says.
"We saw One Direction and James Bond scents generate market growth, but now we aren't seeing as many celebrity launches."
There might be fewer celebrities cologne-ising the shelves (sorry), but Smith says the market is still strong.
"It's definitely not the end of celebrity fragrance, we do still believe there is a place in the market for them," she says.
"Customers still want celebrity perfumes as long as it is the right celebrity, and the right scent."
The right scent, of course, is the crucial part.
When he was the NYT's scent critic, Burr famously awarded Britney's Midnight Fantasy four stars, implying there should be no snobbery about celebrities if the smell itself is good.
He cites Sarah Jessica Parker's Lovely as one of the best of the celebrity fragrances he came across and is now even developing his own, called You or Someone Like You.
Fisher says: "I think overall what we're seeing is the polarisation of the fragrance market.
"At one end, consumers look for value for money, they go for promotions and maybe celebrity fragrances. But at the other, consumers are becoming more selective.
"Rather than buying 10 times a year maybe they buy five times a year. They're spending the same amount of money but going towards a more niche or premium offering."
The fragrance industry was worth £1.25bn in the UK last year, and shows no signs of slowing any time soon.
But if the current sales trends continue, there could well be far fewer famous faces plastered across perfumes in the future.
Expect your next birthday present to be a bottle of Jean Paul Gaultier rather than Justin Bieber.
Celebrity fragrances: A brief history
- Audrey Hepburn and Joan Crawford famously endorsed fragrances by Givenchy and Estee Lauder respectively, while Catherine Deneuve became the face of Chanel no 5 in the 1970s
- Sophia Loren was one of the first to have a fragrance of her own - releasing Sophia by Coty in 1981
- Elizabeth Taylor later released her own range of perfumes, and her 1991 White Diamonds scent went on to become the best-selling celebrity fragrance of all time
- But Jennifer Lopez's Glow, released in 2002, was a turning point - inspiring hundreds of famous faces to get in on the act
- The trend made its way to the UK, with British celebrities such as David Beckham, Alesha Dixon, Little Mix and Tulisa all releasing their own fragrances.
- In 2016, celebrity fragrances made up 4% of the prestige fragrance market, with fashion brands like Boss or Armani making up 53% and around a third coming from couture brands like Chanel or Prada